# How can you lift a marble off a table without touching it with your hands?

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Is it magic? No - it’s physics! Use circular motion to overcome the force of gravity and amaze your family!

## What You Need

• Paper or styrofoam cup
• Small marble
• Utility knife or sharp scissors
• Table

## Safety First!

Be very careful using sharp cutting tools or have an adult help with cutting the bottom off the cup.

## What To Do

1. Remove the bottom of the cup by cutting it off. Ensure that the open bottom of the cup is completely round and fairly smooth.
2. Place the marble on a table.
3. Place the cup (now with the entire bottom removed) over the marble. (See photo).
4. Attempt to lift the marble off the table by using only the cup.

With the marble resting on the table, place the bottom of the cup over the marble so that the marble touches the side of the cup. Move the cup in a circular motion to make the marble spin around along the inner edges of the cup.

Continue moving the cup as fast as you can in a circular motion and eventually the marble will rise up higher inside the cup’s wall. While the marble is spinning inside, begin to lift the cup and marble gently off the table.

In order for the marble to come up off the table, it must overcome gravity, which has a constant force. Once the marble starts to spin, it’s held against the walls of the cup by centripetal force. The marble wants to move in a straight line, but the rounded edges of the cup are forcing it to move in a circle. Centripetal force is the force that pushes or pulls an object inward and forces it on a circular path. If the centripetal force is greater than the force of gravity, the marble will lift off the table. Once moving, the marble will continue to travel along the inside of the cup because of its inertia (the tendency of moving objects to stay in motion).

Centripetal forces are at work in many everyday interactions. When a car tilts on a turn, it is due to centripetal forces. A rollercoaster is forced through a loop by the tracks applying centripetal forces on the coaster cars. Earth’s gravity exerts centripetal force on communication satellites, which keep them in orbit and prevent them from flying off into space.

For more information on this topic check out these Let's Talk Science resources:

• Orbital Mechanics (Backgrounders) - This backgrounder helps explain the laws of Orbital Mechanics, including Newton’s Laws.